The subject invention is directed to battery separators which exhibit a high degree of conductivity and inhibition to dendrite formation and which are capable of being formed in an economically improved manner.
Storage batteries, in general, utilize either acid or alkaline electrolyte with compatible electrode systems. The term "acid battery system" or "alkaline battery system", as used in the present application, refers to battery systems which utilize, respectively, an acidic or an alkaline solution as the electrolyte. An example of an acid battery system is lead acid batteries which are in common use, which examples of alkaline battery systems are those which use silver-cadmium or nickel-zinc electrodes in an alkaline solution such as an aqueous solution of potassium hydroxide.
Because of their high energy density, alkaline batteries, such as nickel-zinc secondary alkaline battery system, have great potential for replacing the more conventional lead acid battery system in a number of terrestrial applications. However, extending the cyclic life of the battery beyond that presently attainable and reducing the cost of the cell components are required criterias which must be met to make the alkaline battery system an effective energy source.
Battery separators are recognized as a key component in attaining an extended battery life and efficiency. Separators are located between plates of opposite polarity to prevent contact between the plates while freely permitting electrolytic conduction. Contact between plates of opposite polarity may be due to imperfections in the plate structure, such as warping or wrinkling of the plate. Such macro deformations are readily inhibited by any type of a sheet material which is coextensive with the plates and is capable of permitting suitable electrolyte passage. Contact may also occur by formation or dendrites or localized needle like growths on an electrode, such as zinc dendrites formed on the zinc electrode in an alkaline nickel-zinc battery system. Separators which are commonly used today are in the form of sheet structures which during formation normally have pores and imperfections of sufficient size to readily permit dendrites to bridge the gap between electrodes of opposite polarity and, thereby, short out the battery system and reduce the battery life.
Various non-elastomeric polymers have been used for forming separators. The term "elastomeric" or "elastomer", or "rubber", as used in the present application refers to polymer materials which are capable of exhibiting a high degree of elongation and recovery. Elastomeric materials are distinguished from other polymeric materials, such as polyethylene, polypropylene, polystyrene and the like which are not capable of exhibiting such stress/strain recovery properties.
U.S. Pat. No. 3,351,495 teaches that certain non-elastomeric polyolefins, such as polyethylene and polypropylene, can be compounded with filler and plasticizer to form a sheet material which, after extraction of some or all plasticizer, forms a microporous matrix suitable as a battery separator. The required use of a high amount of plasticizer and the needed extraction step to form a suitable separator material is costly and, in certain instances, produces irregular results. Separators formed from polyolefins, such as polyethylene, have been irradiated in attempts to increase the structural integrity of the formed sheet material. The resultant crosslinked material, when used as a separator, have been observed to exhibit high electrical resistance and, therefore, generally detract from the formation of effective and efficient battery system.
More recently, certain rubber materials, including synthetic rubbers, have been used in the preparation of battery separators. These rubber materials are solvent cast onto a highly porous substrate support sheet which is normally formed from cellulose of asbestos material. The process of forming such composite separators is both complex and costly and requires removal of the casting solvent and extraction of plasticizer contained in the rubber to form a microporous membrane. Further, these separators are normally of a thickness which reduces the efficiency of the formed battery system.
A battery separator which is capable of increasing the efficiency of a battery system is highly desired. It is generally agreed such a separator should be in the form of a thin, microporous sheet which is resistant to degradation by electrolyte solution, exhibits a high degree of inhibition to dendrite formation and dendrite growth, and has a high degree of electrical conductivity. Further, the battery separator should be of a composition which is capable of being processed and formed into thin microporous sheet material in an efficient and cost effective manner.